Biometric identification systems frequently encounter situations where the body part that is used to establish a biometric identity has been damaged. For example, a friction-ridge surface of a finger (a fingerprint) may be damaged by an accidental burn or abrasion to the finger. Intentional damage to a finger may also impede the ability of a biometric identification system to make a proper identification. Intentional damage may be inflicted in order to prevent identification, for example by abrading, burning or etching the friction-ridge surface. As an example, in Europe people applying for political asylum have burned off their fingerprints in order to avoid having their criminal records discovered. In other situations, terrorists have abraded and acid etched their fingerprints to avoid being identified by security personnel. There is a need for a biometric identification system that is not wholly dependent upon an undamaged friction-ridge skin surface in order to make an identification.
In situations where the friction-ridge surface has been damaged, a biometric identification system may be presented with specimens that are less than optimal for enrollment into a database or for identification purposes. In these situations, it may be advantageous to be able to use other physiological structures that are less susceptible to damage than the friction-ridge surface of the skin. The present invention seeks to utilize physiological differences in sub-surface tissue components. For example, by mapping the valleys between dermal papillae, it is possible to predict what the undamaged friction-ridge surface would look like.
The dermal papillae are small, nipple-like protrusions of the dermis that reach into the epidermis. The papillae bring nutrients and oxygen to the lower layers of epidermal cells. In addition, papillae nourish hair follicles and allow sweat to come to the skin surface to aid in cooling the body.
Rows of papillae form ridges and valleys. The papillae-valleys extend to a substantial depth into the skin. When covered with epidermal cells, the ridges and valleys of the papillae create patterns on the skin, which are commonly called friction-ridges. Fingerprints are the friction-ridges that appear on the fingers. For purposes of illustrating the invention, we will focus on fingerprints, but it should be recognized that the invention may be used with other friction-ridges.
The papillary ridges and valleys develop sometime before birth, and the resulting friction-ridge surface pattern does not change—except to grow larger. Further, the friction-ridge surface pattern is unique for each individual, and can therefore be used for identification purposes.
The papillae are formed from materials that differ in composition from structures surrounding the papillae. The differing compositions allow for detection of the papillae surface by various techniques, and ultimately an image of the papillae surface may be generated. For example, differences in tissue optical opacity and color allow for imaging with optical and infrared techniques. Differences in conductivity may allow for imaging with radio frequency and capacitance techniques. Differences in ultrasonic impedance allows for an ultrasound system to differentiate between the various physiological parts of the skin, including the papillae.
In this document, the term “impedance” is used to refer to the property of a material which resists the propagation of a longitudinal energy wave, such as an ultrasound wave. The impedance, Z, is defined as Z=r·c, where r is the material density, and c is the longitudinal propagation velocity of the energy wave in the material. Propagation of the energy wave is dependent partly on the particle mass (which determines the density of the material) and partly on the elastic forces binding the particles together. A fraction of the energy pulse may be reflected whenever there is a change in impedance. The larger the change in impedance, the larger the fraction of energy reflected. The fraction of energy reflected as a result of differences in impedance between two materials can be calculated by the equation, R=((Z1−Z2)/(Z1+Z2))2, where R is the fraction of the energy reflected, Z1 is the impedance of the first material, and Z2 is the impedance of the second material.